Solder products for use in the manufacture of electrical and electronic equipment come in a wide variety of shapes of forms, such as ingots, rods, wires, pastes, and shaped forms such as pellets and washers. Most solder products consist entirely of a solder alloy which melts during a soldering process. However, when performing soldering using a solder product made solely of a solder alloy, it is sometimes difficult to maintain a desired spacing between two members to be soldered to each other.
For example, in die bonding, in which solder is sandwiched between a semiconductor element and a substrate and then the solder is heated to cause it to melt, the weight of the semiconductor element acting on the molten solder may cause a considerable amount of the molten solder to be squeezed out of the space between the semiconductor element and the substrate and reduce the size of the space. The reduction in the amount of solder remaining in the space between the two members results in a significant decrease in the bond formed by the solder.
To avoid this reduction in the spacing between members being soldered, solder products containing metal particles having a higher melting point than the solder (referred to below as metal particles) have been developed. When a solder product with dispersed metal particles is disposed between two members and heated, the solder melts but the metal particles remain substantially unmelted, so the metal particles can maintain a suitable spacing between the two members to hold a sufficient amount of molten solder to ensure a reliable joint between the members. Both solder pastes and solder pellets containing metal particles have been developed.
A solder paste with dispersed metal particles can be prepared simply by adding metal particles to a previously manufactured solder paste and stirring. The manufacture of solder pellets with dispersed metal particles is more complicated. First, metal particles are charged into a large melting pot containing molten solder, then the molten solder and the metal particles are stirred with a spatula to disperse the metal particles, and then the molten solder and dispersed metal particles are scooped from the melting pot with a ladle and cast into a mold for manufacturing a billet. The resulting billet is then extruded into the shape of a plate by an extruding apparatus, the plate is subjected to rolling, and then the rolled plate is punched with a press to form pellets of a desired shape.